In inline math mode the integral/sum/product lower and upper limits are placed right of integral symbol. Similar is for limit expressions. If you want the limits of an integral/sum/product to be specified above and below the symbol in inline math mode, use the \limits command before limits specification.

LaTeX code

Output

Integral $\int_{a}^{b} x^2 dx$ inside text

Improved integral $\int\limits_{a}^{b} x^2 dx$ inside text

Sum $\sum_{n=1}^{\infty} 2^{-n} = 1$ inside text

Improved sum $\sum\limits_{n=1}^{\infty} 2^{-n} = 1$ inside text

Moreover, adding \displaystyle beforehand will make the symbol large and easier to read.

On the other hand, \mathlarger command (provided by relsize package) is used to get bigger integral symbol in display.